Currently-implemented OFDM networks use either a frequency division duplexing (FDD) or a time division duplexing (TDD) scheme. In FDD-based communication systems, the uplink uses a different frequency band from the downlink. Typically, these systems make use of symmetric spectrum allocation, i.e., the bandwidth allocated to the downlink is the same as in the uplink. Disadvantages of this approach include the use of a fixed spectrum band that is inflexible to reconfiguration of the uplink and downlink bandwidth to support different data rate and capacity requirements for different classes of service. In TDD-based communication systems, the uplink and the downlink share the same frequency band, but the uplink and downlink transmissions occur at different times. Thus, these systems provide flexible use of the spectrum between uplink and downlink. However, disadvantages with this approach include delays in transmissions due to having to wait for the allotted transmission time and lower link budgets due to mobile stations not being able to transmit continuously.
Therefore, there is a need in the art for an improved method for allocating sub-carriers for communication in an OFDM network. In particular, there is a need for a system that provides spectrum flexibility to accommodate downlink and uplink traffic asymmetry and that does not suffer from delays and a lower link budget.